A high molecular weight, fatty acid- and SDS-sensitive protease named ingensin was purified from rat brain in this study. The enzyme purified from rat brain has the same biochemical properties as those purified from other tissues, e.g., porcine skeletal muscle, human placenta, and rat liver in our laboratory, and rat skeletal muscle and bovine pituitary gland in other laboratories, independently. Immunoblot bands were detected in the same positions as those in the case of ingensin from rat liver. In addition, its topographical distribution was studied in rat brain and muscle by means of the immunohistochemical method. The cytoplasm of motor neurons of the spinal cord, pyramidal cells, and granular cells of the hippocampus, Purkinje cells, and glial cells were stained. Axons were stained. The cytoplasm of muscle was also stained, especially that of type 2 fibers.